The ammonia decomposition occurs on the dynamic Ru nanocluster surface on the graphitic nanofibre support. The activity of the catalyst depends neither on mean particle size nor on maximum Ru atom dispersion since in hydrogen and ammonia atmospheres the Ru atoms form isolated atoms, few-atom aggregates, amorphous islands, and compact nanoclusters. In this work the ammonia-induced step-edge formation of Ru nanoclusters on graphitic nanofibers is found out at 450~$^\circ$C. Hydrogen activates the Ru atoms creating a Ru inventory: the total amount of single atoms, dimers, trimers grows up from 39\% to 87\% while the fraction of clusters falls from 61\% to 13\%. Ammonia recombines some part of the inventory into clusters reducing their amount up to 60\% and bringing clusters back up to 40\% after 3~h of exposure. On the cluster level an amorphous island with $N_{\rm at}=175$ and $S_{\rm FP}=2.35$~nm$^2$ is transformed into a four-layer crystalline cluster with $N_{\rm at}=231$ and $S_{\rm FP}=3.13$~nm$^2$. Long-term ammonia treatment yields a five-layer crystalline cluster with $N_{\rm at}=374$, $S_{\rm FP}=3.62$~nm$^2$, and 103.3 atoms/nm$^2$ staying below the approximate 4~nm$^2$ area limit. Thus, the catalyst becomes active due to the ammonia-induced transformation of the mobile Ru inventory into the compact clusters with numerous steps on graphitic carbon. Stepped nanoclusters provide a stable hydrogen production, not the hydrogen-induced atom reservoir or particles' growth.

ammonia decomposition,ruthenium,graphitic nanofibres,nanoclusters,step edges,hydrogen production,identical-location microscopy